A positron emission tomographic apparatus (PET) uses positron-emitting radionuclide to emit a positron, which then annihilates when colliding with an electron in a material and emits two gamma rays in exactly opposite directions, and detects the emitted gamma rays to obtain a nuclide distribution image (non-patent reference 1). A scintillator detector is generally used as a gamma ray detector for the positron emission tomographic apparatus. The scintillator detector receives light emitted by gamma ray detection with several photomultipliers, and determines with which scintillator the gamma ray was measured based on the intensity ratio of each light. The spatial resolution of a positron emission tomographic apparatus adopting this principle is several mm at the maximum.
Namely, spatial resolution of 1 mm or higher comparable to that of an X-ray computed tomographic apparatus (CT) cannot be obtained with a scintillator detector.
As a semiconductor detector for detecting light or particle beams, a semiconductor position detecting element (Patent reference 1) and a detector using a cadmium-telluride (CdTe) crystal, which has high absorption effect for gamma rays, are known (Patent reference 2). The CdTe detector has CdTe crystal semiconductor plates, on the surface and rear surface of which electrically conductive electrodes are formed, and electric signals are detected via an amplifier. The use of this semiconductor detector could downsize the detector, and even a 1 mm-size detector could be achieved. Consequently, it would be possible to achieve a semiconductor detector having the spatial resolution of 1 mm or lower when two or more such detectors are arranged. However, to obtain spatial resolution of 1 mm or lower by arranging ten 1 mm2 detectors in the vertical and horizontal directions respectively, a total of 100 amplifiers are necessary. Since an enormous number of amplifiers are necessary, the detector of the above composition is not feasible.
To overcome the disadvantage described above, patent reference 2 discloses a detector that uses a 20 mm2 semiconductor plate, for example, on whose rear surface an electrically conductive electrode is formed, and two-dimensionally detects gamma ray positions within the semiconductor plate using the ratio of electrical signals from four corners of electrically conductive resistive electrodes. With this detector, detection is realized when just four amplifiers are provided.
FIG. 12 illustrates the semiconductor 2D position detector 50 disclosed by patent reference 2. With this semiconductor 2D position detector 50, a thin semiconductor crystal plate 51 is made of a CdTe crystal. An electrically conductive resistive electrode 52 is formed on one of the surfaces of the thin semiconductor crystal plate, and, an electrically conductive electrode 53 is formed on the other surface.
To make the semiconductor crystal plate 51 made of a CdTe crystal into a Schottky type detector, an indium electrode 52 is formed on one surface, and a platinum electrode 53 is formed on the other surface. The indium electrode is made to have electrically conductive resistivity by depositing indium thinly i.e. 600 Å for example. This allows the indium-deposited surface of the semiconductor crystal plate 51 to have electrically conductive resistivity and operates as a Schottky type radiation detector.
At each of the four corners, namely A, B, C and D, of the semiconductor 2D position detector 50 a terminal is mounted. Each of the terminals is connected to an amplifier circuit 55, and the output signals generated at the four terminals, namely voltages VA, VB, VC and VD, are used to find the calculated positional coordinates (X, Y) (hereafter called temporary positional coordinates) of the gamma ray on the semiconductor plate 51 as a function of VA, VB, VC and VD, (Patent reference 2).
FIG. 13 illustrates the irradiation positions of radiation 57 in a conventional semiconductor 2D position detector 50. As shown in the figure, in order to examine the accuracy of detected positions of the conventional semiconductor 2D position detector 50, an alpha ray is irradiated by using a 241Am source to fifteen points of intersection 58 (X=7, 11, 15, 19, 23 and Y=6, 10, 14).
FIG. 14 shows the result of position detection conducted using the conventional semiconductor 2D position detector 50, namely temporary positional coordinates 59 calculated using output signals V1 to V4 generated at the four terminals. Despite that radiation was applied onto the surface of the semiconductor position detector 50 at equally spaced intervals, the positions expressed as temporary positional coordinates 59 and the actual radiation positions detected were found to be non-linear and asymmetrical. Such highly distorted distribution was insufficient to identify the detection positions of the radiation 57.
Patent reference 3 discloses a semiconductor 2D position detector that has a resistive layer on a semiconductor substrate having sides of a circular, instead of square, and it is described that the position of incidence of γ-rays without distortion can be detected by detecting current with the four vertices formed with each side used as output electrodes.
Patent reference 4 discloses a radiation position detector that detects 2D positions using a resistor connected by cascade connection. However, it has a very complicated circuit structure including two amplifiers to be connected to resistors, four A/D converters to be connected to the amplifiers, two position calculators, an adder, an amplitude discriminator, and a control signal generator.
Patent references 5 and 6 disclose the formation of a structure with linear resistance wires installed in parallel (Patent reference 5), and a matrix structure (Patent references 5 and 6), using CdTe, CdZnTe or BrTl as a semiconductor substrate used for a semiconductor 2D position detector. The detecting elements in Patent reference 6 are arranged in a 2D matrix state. In this case, a switching device and an amplifier must be installed for each of the detecting element, which results in a circuit structure as complicated as that of Patent reference 4.
Patent reference 7 discloses a semiconductor 2D position detector using Si as a semiconductor substrate with Al electrodes formed in stripes on the surface layer.